1. Field of the Invention
The present invention relates to a zoom lens suitably used in an image pickup apparatus such as a digital still camera or a video camera.
2. Description of the Related Art
A zoom lens for use in an image pickup apparatus (camera), such as a video camera or a digital still camera, including a solid-state image pickup element is required to have a compact lens system and a high optical performance.
In a camera including a solid-state image pickup element, a zoom lens needs to have a relatively long back focus because optical components, such as a low-pass filter and a color correction filter, are disposed between the rear of the lens and the solid-state image pickup element.
In order to avoid color shading, a color camera including a solid-state image pickup element for a color image is required to include a zoom lens having a good telecentric characteristic on the image-side.
Further, in order to reduce the thickness of an image pickup apparatus by using a so-called retractable zoom lens, it is known that protrusion of lens units from a camera body is reduced by decreasing the distances between the lens units in the zoom lens while an imaging operation is not performed. In this case, for efficient thickness reduction, it is necessary to reduce the number of lenses that constitute the lens units so as to reduce the thicknesses of the lens units.
If the moving amounts of the lens units during zooming and focusing are large, the total length of the zoom lens increases, and it is difficult to reduce the retracted length and to use the lens units in the retractable zoom lens. This tendency becomes more pronounced as the zoom ratio of the zoom lens increases.
A negative lead zoom lens in which a lens unit having a negative refractive power leads is known as a zoom lens having a relatively long back focus.
U.S. Pat. Nos. 6,545,819 and 6,498,687 disclose a compact three-unit zoom lens in which a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, and a third lens unit having a positive refractive power are arranged in that order from the object side to the image side. The zoom lens is telecentric on the image side.
U.S. Pat. Nos. 7,019,911 and 7,215,483 and Japanese Patent Laid-Open No. 2006-65034 disclose a three-unit zoom lens that is provided with a high zoom ratio of three or more by moving all lens units during zooming.
U.S. Pat. No. 4,838,666 and Japanese Patent Laid-Open No. 7-261083 disclose a wide-angle three-unit zoom lens that has a long back focus and that is telecentric on the image side.
In the above-described three-zoom lenses, the second lens unit serves functions as a variator. For this reason, in order to reduce the total size of the lens system and to achieve a high optical performance over the entire zoom range, it is necessary to reduce aberration changes during zooming in the second lens unit. For that purpose, it is important to appropriately set the refractive power and magnification of the second lens unit serving as the variator and the lens configuration.
In addition, it is important to appropriately set the imaging magnification of the third lens unit in order to increase the zoom ratio.